Dromaeosaurid Archaeopteryx

Gregory S. Paul

Getting ready for the Soc Avian Paleo & Evol conference on Mesozoic birds in June in DC has caused me to restudy Archaeopteryx and protoavian (sensu Paul 1988 PDW) theropods. With all the specimens old and new now available, virtually the entire skull and skeleton is now known for Archaeopteryx. Conclusions are as follows.

Archaeopteryx truly is a flying theropod that shows virtually no avian characters not observed in other theropods (the large, distal, fully reversed hallux may be the only exception). Contrary to reports otherwise, there is no evidence for avian skull kinetics, and the postorbital bar was probably complete. The palate and braincase are entirely theropodian in structure.

Archaeopteryx is not only a theropod, it is a dromaeosaur because it shares a number of detailed characters only with dromaeosaurs. Some other characters are found only in the two forms and some basal birds. Some of the Archaeopteryx-dromaeosaur characters are as follows.

• Nasal depressed nasal and snout upturned
• Dorsal process of maxilla almost reaches preorbital bar
• Preorbital bar slender & straight preorbital in lateral view
• Dorsal depression on the ectopterygoid
• Diamond shaped supraoccipital
• Strongly twisted paraoccipital process (noted by Currie)
• Highly modified tail with hyperdorso-flexible base (condition approached in troodonts)
• Middle finger most robust
• Ilium parallelogram shaped (also basal birds)
• Pubic peduncle very large & reversed
• Ilio-pubic articulation inverted V shape
• Pubic shafts are flat plates oriented 140 degrees to each other

In addition, the foot of Archaeopteryx is functionally two toed, with a short toe II that is hyperextendable.

Many Cretaceous theropods - dromaeosaurs, troodonts, oviraptors - are more bird-like than Archaeopteryx in many respects, and have a shoulder girdle that is similar to secondarily flightless birds. All this suggests that avian flight first evolved in arboreal theropods (where they developed big brains and forward facing eyes, features not found in flying insects and pterosaurs), and that some of the flying theropods lost flight. Not knowable at this time is whether Archaeopteryx was a member of the true bird clade, or was an independent experiment in flight, or where theropods end and birds begin, among other matters.